The basic components of corn kernels are bran and tip cap (“fiber”), endosperm (“starch”) and the germ (“lipid”). The tip cap is the outer shell and coating of the germ. The bran is the fibrous membrane that holds the endosperm together. The endosperm is composed of starch bodies that forms the major portion of the kernel. The germ contains the majority of the lipids within the kernel. Corn comes in a number of varieties each defined by its typical or average composition of endosperm, germ, tip cap and bran fractions. One widely used variety is #2 corn having a representative kernel composition as presented in the following table:
% of Kernel% Lipid ContentWhole Kernel100% 2.6% typical(Higher in some)Endosperm81%<2%Germ12%~82% Tip Cap 1%<1%Bran 6%<2%
Currently, there are many methods to mill or fractionate whole kernel corn and separate these components out into three distinct process streams, a germ steam, an endosperm stream and a bran/tip cap stream.
By fractionating the kernel into these fractions or stream fractions, each of the fractions can be processed more easily and with better yields. The separate streams can be used as a feed stock for various processes and products. For example, the bran stream can be burned as fuel or further extracted to yield pharmaceuticals and/or nutriceuticals. The endosperm stream with its high starch content can be processed into many things, e.g., ethanol and sugar being only two. The germ stream is mainly further processed for its lipid content.
The various methods of fractionation known to those skilled in the art can generally be divided into two primary processing categories, dry milling and wet milling. Wet milling is generally regarded as a very effective technique for producing distinct process streams of relatively high purity. This effectiveness is evident in the fact that the starch, the lipid, and fiber content in each of these streams are very concentrated within its relative stream. While wet milling typically will yield higher purity fractionate streams compared to dry milling, it also is comparatively more expensive than dry milling. Thus, most operations using wet milling are doing so because they need or require these distinct streams as pure as possible. Wet milling could be considered to be or referred to as a “chemical” process.
Dry milling on the other hand is less expensive but does not result in the same degree of separation into distinct process streams; and therefore does not provide the purity required for some operations. A consequence can be the loss or waste of portions of the fractionate, e.g., more germ in the bran fraction represents a potential loss of lipid values. Dry milling typically could be considered a “physical” process.
It would be desirable to provide a dry milling process for corn kernel fractionation that provides improved purity of process streams, and even more desirable to provide a dry milling process that can provide purity levels approaching those achievable with wet milling. Further it is desirable to provide a fractionation process that can yield mixed processed stream of desired composition or degree of mixing for select operations.